If Michelle Rodriguez and Gina Carano share a single regret, it's having not risked spinal injury together to film a key sequence for the thriller "Fast & Furious 6" last year. The action actresses get downright aw-shucks-y — wistful, even — discussing the missed opportunity to careen over a handrail locked in combat and tumble down a grotty stairwell in London's Underground: one of the most over-the-top scenes from one of this summer's most hotly awaited movies, which hits theaters late Thursday.

Stoking a form of discontent that's unique to strong-willed women who put up their dukes in mega-budget thrillers, "Fast 6" producers insisted Carano and Rodriguez leave the stairwell stunt to professional stand-ins for fear that a busted tooth or torn rotator cuff would shut down the movie.

Arriving with the smack of a swift uppercut, the square-off between Rodriguez and Carano's characters showcases what might be the grittiest, rawest woman-on-woman brawl in mainstream cinema. Call it an XX chromosome counterpart to the fights in the "Bourne" action-movie franchise: nasty, brutish and often painful to watch precisely because the scenes seem so real compared with other film violence. And yes, that includes the trailer-demolishing kung-fu contretemps between Uma Thurman and Daryl Hannah in "Kill Bill, Vol. 2."

Over the course of the two-minute punch-up, Carano and Rodriguez exchange gut-kicks and knee strikes, an elbow to the larynx, a head-butt and, in Carano's case, one well-timed Flying Arm Bar. To execute that acrobatic submission move, the former mixed martial arts champ leaps up, catches Rodriguez's character's head in a leg scissor and flap-jacks her to the floor.

"We're doing Part 6 of a franchise. But I don't want to rest on that," said Justin Lin, who has directed the last four installments of the series, which has taken in $1.6 billion worldwide. "Why not make it be the best female fight you've seen in cinema?"

Rodriguez took a philosophical view on filling a niche in Hollywood that was once a male reserve. "Guys have the best scenes dating back to the beginning of action films," the actress said. "Girls don't have that. It's an empty … lane."

In a summer crammed with bombastic movie eye candy, the woman-on-woman, mano-a-mano action offers counterprogramming of a sort: a comparatively lo-fi spectacle. Even within a film featuring a muscle car jumping out of a crashing plane and a tank chase on a highway in Spain, the "Fast 6" lady smack-down is distinguished by its lack of computer-generated imagery.

For the sequence when Carano's hell-for-leather government agent character chases, then tangles with Rodriguez's Letty — an outlaw driver from the original "Fast & Furious" team who was presumed dead in "Fast 4" but who has been mysteriously resurrected as a ruthless terrorist — the production let flying fists do all the talking. "There's something special when you get two physical people on film, and it's actually them instead of CGI, or action that's sped up or slowed down," Carano remarked. "And I think there's something very beautiful about two females going at it. There's a different energy."

It helps when the on-screen nemeses can back up their punches with real-life experience.

Carano, 31, a former stunt professional on the TV game show "American Gladiators," was ranked among the top mixed martial arts fighters in the world before she retired to work in Hollywood. Her breakthrough: a starring role in the Steven Soderbergh-directed 2011 thriller "Haywire." Rodriguez broke into moviedom with the 2000 indie boxing drama "Girlfight," a role for which she packed on muscle and extensively trained to portray a sort of teen girl version of Rocky. The 34-year-old actress has honed her tough-as-Kevlar persona in movies including "Avatar" and "Battle Los Angeles" as well as two previous "Fast" installments.

Dana Polan, professor of cinema studies at New York University, explained that modern movie fights between women draw their power to entertain from two contradictory sources: post-feminism and exploitation.

"After feminism, there's a way in which women have been empowered to become action heroes," he said. "And for men watching women fight in action movies, there's a strange mix of sadism and sadomasochism.

"On the one hand, men like to see women beat up on women," he continued. "It fulfills a stereotype that women in relation to other women are catty. They are rivals. And it allows a display of the female body that, even in moments of violence, is sexy in its own way."

With input from Rodriguez, however, the "Fast 6" fight evolved away from what was essentially a catfight on steroids (as scripted by Chris Morgan) to play more like an epic street fight.

"Originally in the script, it was a lot more 'Terminator'-esque — too far-fetched to be believed," Rodriguez said between bites of kale salad at a Venice lunch spot. "Things just happened so quick and then I'm on top? Justin and I had to bust our booties to get it more realistic. I was like, 'This [woman] needs to kick my ass!'"

First came two months of rehearsal during which the actresses choreographed their scenes until the attacks and blocks became second nature. Carano remained aware she was responsible for Rodriguez's safety. So the former pro put in extra practice to ensure her strikes would look credible but not land so hard as to concuss her costar.

"I had to take everything I was taught and reverse it," Carano said. "In training in mixed martial arts, you're taught to keep all your emotions inside and then just exude it through your power. With acting, you bring all your emotions to the outside and pull back your power. It's kind of like opposite day."

Under the direction of Olivier Schneider, the French fight coordinator credited in "Taken" and "Safe House," the scene (and a later "rematch" aboard a cargo plane) unfolds as a battle royale between two accomplished yet differently skilled combatants.

"We were very specific," said Schneider. "Michelle is not a professional fighter. She's a street fighter — she's just trying to escape and survive. Gina was a pro MMA fighter; she's supposed to play a cop with fight training. We were all on the same page to do something brutal and spectacular but realistic."

With the actresses' growing sense of camaraderie came a physical comfort level that compelled Rodriguez to take risks. On set, she began badgering Carano not to pull her punches.

"She was so adorable about it, like, 'Go for it! Give it to me harder!' And I was like, 'You don't want that,'" Carano recalled with a laugh. "'We have to work tomorrow.'"

In the end, Carano and Rodriguez take opposing positions on what the future of female film fighting may hold.

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